The present invention generally relates to transformation of a color image from one color space to another, and more particularly to a color correction system for transforming the intensities of input RGB data of a color scanner into the densities of output YMC colors for a color printer or display. The color correction system is applicable to image forming systems such as a color facsimile machine and a color copying machine.
Conventionally, there have been proposed color correction methods for transforming the intensities of input RGB data of red (R), green (G) and blue (B), into the densities of output YMC colors of yellow (Y), magenta (M) and cyan (C). The input RGB data is generated from an original image by a color scanner, and the input RGB data is input to a color correction circuit of an image forming system. In the image forming apparatus, the color correction circuit generates the output YMC signals from the input RGB signals through color transformation, and, the quantity of ink of each of yellow, magenta and cyan colors used in printing a color image by a color printer is controlled in accordance with the output YMC signals of the color correction circuit.
Among the above mentioned color correction methods, there are a linear masking method, a nonlinear masking method, and a memory mapping method. In the linear masking method, color correction is performed to eliminate only linear distortions from the image. The nonlinear masking method is to eliminate linear and nonlinear distortions from the image. In the memory mapping method, the transformation of a color image is performed using a memory in which combinations of output YMC values corresponding to each of combinations of input RGB values are stored.
In the case of the linear masking, the implementation to hardware is easy, but it is difficult to produce accurate output YMC color signals. In the case of the nonlinear masking, highly accurate color signals can be produced as outputs, but many multipliers are required in the color correction circuit, and the hardware size becomes large. Generally, when either the linear masking method or the nonlinear masking method is used, it is difficult to accomplish flexible color modification necessary for a color copier machine to output a color image with the desired image quality.
When the memory mapping method is used, accurate color correction and flexible color modification can be achieved. However, the implementation to hardware is difficult because the memory of the color correction system requires a large capacity to store all the combinations of densities of output YMC colors corresponding to each combination of intensities of input RGB data.
Moreover, there has been proposed a color correction circuit that is relevant to the above mentioned color correction methods. In this color correction circuit, when a conventional UCR process is performed in the color correction, the output YMC color signals are produced through matrix computation with a matrix of color difference values between the minimum signal (black) and the input color signals.
It is desirable to produce output YMC colors for the achromatic area of YMC color space with high accuracy of color reproduction. However, generally, it is difficult to make color differences between the original image and the reproduced image for the achromatic (black-and-white) area negligible. In order to achieve a highly accurate color reproduction for the black-and-white area, there has been proposed an improved color transformation method. For example, Japanese Laid-Open Patent Publication No.64-47174 discloses such a color transformation method.
In the color transformation method disclosed in the above mentioned publication, the input RGB values are separated into color components and black-and-white components, and the color transformation to produce output YMC colors is performed separately for each of the two components, so as to achieve accurate color reproduction for the black-and-white areas of the image. However, in this conventional method, it is necessary that different sets of coefficients be used to perform matrix computations for each of divided color areas. For this reason, relatively large gaps of the color densities appear between the different color areas, and the continuity of change of the color densities is lost. Therefore, due to the use of different coefficients of the matrix computation for different color areas, the above color transformation method has a problem in that the quality of the reproduced image becomes poor.